Our results contravene the simple intuition that directionality should impede cooperation. Even though it disrupts reciprocity, we have proven analytically that cooperation can be favored to evolve in populations with purely directional social interactions and directional spread of behavior. This analysis is based on the same theoretical framework that has been widely applied to study the emergence of cooperation in undirected social structures [1]}, [2]}. Our results rest on the simple intuition that cooperators tend to have more cooperative neighbors than defectors do – and that, in fact, this effect is maximized when an intermediate proportion of edges in the population are uni-directional.
